I'm curious what the white stuff is and why this is a preferred look for some people?

I know zero about board building, but I know you have a base material that is foam and you wrap it in fiberglass/carbonfiber and wet is out with epoxy, then typically I believe you apply a hot coat which is a different resin that drys harder for sanding and polishing.

Where in the above process does this patched white stuff come into play?

In order to save weight and time, they fill the laminate layer with an epoxy puddy in white over the black CF. This allows easier sanding, easy to see when you have sanded thru and removed enough but not too much, white reflects light keeping board cool in sun and prevents uv light degeneration. Then it is probably spray painted with a 2 component urethane that is also lighter, less work and better uv protection than a traditional hot coat.

The raw look has also become popular because you see the construction and processes, which is trend in many other product domains too.

Thanks for the explanation. Is the typical surfboard eps foam a much harder/denser construction that what you find at the hardware store? I'm entertaining the idea for a build but I went and looked at the eps foam at Lowe's and I could easily push my finger into it.

It's not epoxy "puddy" (maybe just a bad translation) but white opaque resin tint that's added to the hotcoat/sanding coat. You can tell it's on the sanding coat as the white pigment isn't in between the fibers of the woven.

Yes you do get a side benefit of having what you'd call a "guide coat" but that's not the primary use. It is primarily there to keep a carbon board cooler.

Sanding coats aren't usually a different resin, but a board builder will more often than not put an additive in to give it more desirable qualities for sanding.

I love Camet Boards and one of the choices you have is a "distressed look" where Brian will actually sand away paint before doing the final clear coats. Makes it a unique piece and distracts theft since it looks old and beat up.

100% sure that board in the pic would have been painted solid white before he sanded in the "distress".

Attached pic is my board where blue is sanded back to expose white undercoat. Its a full carbon board.

Thanks for the explanation. Is the typical surfboard eps foam a much harder/denser construction that what you find at the hardware store? I'm entertaining the idea for a build but I went and looked at the eps foam at Lowe's and I could easily push my finger into it.

Might be misremembering, but I don't think it matters -- the strength is in the composite wrap and the core is there to give it shape.

"Sanding coats aren't usually a different resin, but a board builder will more often than not put an additive in to give it more desirable qualities for sanding."

That's what I mean. ( I build boards). Add glass balloons to epoxy to make a puddy. It's like mayonnaise. Easily sanded and lighter.

Eps from builders is about what board builders in Us call 1lb per ft3. You'll need to get2lb foam. Harder to find.
If your starting out, get a Polyurethane surfboard blank and shape it. It will be way easier to shape, glass and fix if you mess it up. Which you can count on if it's your first board.
Use epoxy not polyester resin. Greenlight surfboard supply has good guide.

I first noticed this paint effect about thirty years ago , on some "Pro" team windsurf boards, at the time most boards were predominately white,
These Carbon copies of production boards were painted white, and quite a few riders had sanded back the paint finish to further lighten the boards, i first saw this done on the underside of the board.
The effect was taken up by manufacturers, as it does two things, shows the board is Carbon construction, (expensive, so you may a well show it off) while also having a paint finish to prevent excessive heat build up, which you get with a black Carbon board,
I have a Mitu Carbon, it's Blue and Orange, factory sanded back possibly for lightness, but more likely cus customers like to show they bought the flash one !

That's what I mean. ( I build boards). Add glass balloons to epoxy to make a puddy. It's like mayonnaise. Easily sanded and lighter.

This. Its microballoons and epoxy that gets pasted over the board to fill any uneveness and pinholes from the epoxy degassing etc. Its easier to sand and gives a really smooth finish and completely seals the board. You can then also add an epoxy pigment and use it as a hotcoat. Those dark patches that you see are the high spots of the carbon fibre showing through the sanding. This is most probably a vacuum job where you get a lot of low and high spots that come from the sandwiching of denser foams like pvc aerex - both of which add strength.